This invention is directed to electrical circuit breakers and, more particularly, to electrical circuit breakers having a trip mechanism controlling the operation of the circuit breaker, wherein the trip mechanism includes a tip bar.
Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 5,705,968; 5,831,503; and 5,341,191. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high-level short circuit condition.
Molded case circuit breakers include a pair of separable contacts per phase which may be operated either manually by way of a handle located on the outside of the case or automatically in response to an overcurrent condition. Circuit breakers include an operating mechanism that is designed to rapidly open and close the separable contacts, thereby preventing a moveable contact from stopping at any position which is intermediate to a fully open or a fully closed position. The circuit breaker also includes a trip mechanism having a means for sensing an overcurrent condition in the automatic mode of operation; a trip bar responsive to the sensing means; and a latch mechanism including a trigger mechanism. During an overcurrent condition, the trip bar responds to the sensing means and releases the trigger mechanism. The trigger mechanism releases a latching and releasing mechanism, which, in turn, opens the separable contacts.
The trip bar of the trip mechanism is rotatable by one or more trip sources to release the trigger mechanism. "Latch load" is conventionally defined as the force required by a test probe at a trip point on the trip bar, such as the actuation point for accessory attachments, to cause sufficient torque about the axial centerline of the trip bar necessary to release the trigger mechanism. At least two other torques are present on the trip bar during a tripping action.
First, there is a frictional torque resisting rotation of the trip bar due to friction between the trip bar and the operating mechanism side plates at the trip bar pivot points. The second torque is due to the load imposed by the trigger mechanism on the trip bar at a loading point. This torque tends to push the trip bar "off latch". The force associated with this torque is dependent upon many variables within the circuit breaker operating mechanism (e.g., biasing spring force, parts tolerance) which are normal manufacturing variables.
With suitable moments, a force (e.g., about 300 pounds) in the operating mechanism may be offset by a relatively small load (e.g., about 30 ounces) where a plunger engages the trip bar, thereby controlling a relatively large force with a relatively small force. As a result, even relatively small position variations in the latching and releasing mechanism may cause significant changes in the direction of the operating force. This, in turn, reflects directly in the corresponding latch load and "shock-out" sensitivity (i.e., the sensitivity of the operating mechanism to a premature release). The corresponding latch load may be subject to a relatively large amount of variation due to the various positions assumed by components of the operating mechanism and the latching and releasing mechanism resulting from: (1) normal manufacturing tolerances; (2) production heat-treating operations; and (3) normal operating variations between latching operations.
Sufficient latch load is required in order to maintain the circuit breaker operating mechanism in the latched position. Too little load may cause the operating mechanism to shock-out. For example, if the corresponding latch load is too small, the operating mechanism may shock-out to a trip position when the circuit breaker handle is moved to the ON position. Also, manual "push-to-trip" operation of the circuit breaker may be adversely affected in the OFF position of the operation mechanism. In such OFF position, the force of the operating mechanism is further reduced because a spring of the operating mechanism may be stretched less with respect to the ON position. In turn, the corresponding reduced latch load may be insufficient to overcome the normal frictional forces within the operating and trip mechanisms. Conversely, relatively large latch loads may inhibit the automatic mode of operation during an overcurrent condition. Too much load may prevent the operating mechanism from tripping after an overload or short circuit event is detected in the circuit breaker trip unit and a trip initiation is begun. Excessive load may also prevent accessory attachments, such as shunt trips or undervoltage releases, from causing the operating mechanism to trip when appropriate.
In conventional practice, a circuit breaker is assembled and the latch load is measured to ensure that it falls within specified limits. The range of these limits tends to be rather wide in order to increase manufacturing yield. If the latch load is out of specification, the usual remedy is to substitute a new trip bar bias spring and/or manually stretch the bias spring, bend it, or cut one or more coil turns from it. In some cases, the circuit breaker trip bar is replaced or scrapped. These remedies all require a certain degree of disassembly and are costly in terms of labor and materials.
One known trip bar includes pivot pockets that receive pivot points on side plates of the circuit breaker. After repeated operation, the pivot pocket can become worn and distorted, thereby shifting the location of the latch relative to the trigger.
There is a need, therefore, for a way to reduce wear in the pivot pockets of the circuit breaker trip bar to improve the operating life of the trip bars.